Triassic fossils of West Texas

Rick Day (USA)

I found my first Triassic fossil when I was about 15 years old on a backpacking trip in Tule Canyon, Briscoe County in Texas. After setting up camp, I walked over to a small, red-coloured hill. On its slope, I found a small, unusual bone fragment approximately 5cm by 5cm by 1cm and I remember being really excited. It was the first petrified bone I had ever found and it seemed strange to me. The bone fragment was flat on one side and had dimples and pits on the other side. It just didn’t look like anything I had ever seen before.

I carried this unusual bone fragment in the glove compartment of my car for years, hoping to find someone who could identify it. However, I was never able to find anyone who knew anything about the fossil. Eventually, I became interested in geology and learned for myself the geology and paleontology of the region where I live. This eventually led to my becoming a science teacher.

In time, I was able to identify my strange little fossil bone as being a scute fragment from the back armour of a Thecodont Reptile called an Ateosaur. (A “scute” is thickened, horny or bony plate that can be seen today on the shells of turtles or on the backs of crocodiles.) For those unfamiliar with Ateosaurs, these were the heavily armored reptiles of the Triassic Period that are, in many ways, similar to the dinosaurian Nodosaurs and Ankylosaurs of the Cretaceous Period.

It turned out my little ateosaur fossil bone fragment wasn’t really all that rare here in West Texas. Fossil fragments from ateosaurs and parasuchians (Phytosaurs), along with a host of other Triassic reptiles, are quite common in some locations. Very rarely, small dinosaur fossils can also be found, such as early small forms of theropods and ornithicians. For the most part, these are poorly understood.

West Texas has a treasure trove of fossil-producing Triassic rock formations. Mostly, these Triassic beds are contained in an area referred to locally as the “Caprock Canyonlands”. These Canyonlands are found on the eastern side of the Llano Estacado, or Staked Plains, and represent the westward erosion of those plains. This erosion reveals itself as large, rugged canyons many hundreds of feet deep. The Canyonland zone is irregular in outline but broadly runs from the Canadian River Valley of the Texas Panhandle southward to the edge of the Edwards Plateau, this distance being about 400 to 500km. The east-west width varies considerably but ranges from just a few kilometers to as much as 40km wide. It is within these canyons that fossil-bearing Triassic rock is found.

Texas Triassic rock belongs to a group of formations referred to as the “Dockum group” and is late Carnian to early Norian in age, being roughly 200 to 215 million years old. There are three formations in the Dockum group. The basal Tecovas formation is overlain by the Trujilla formation and, in some areas, the Cooper Canyon formation overlays the Trujilla formation. All three formations produce Triassic Vertebrate fossils.

Personally, I have more experience of the upper Tecovas and the Trujilla formations. These formations consist of red to maroon-coloured shales and siltstones, grey sandstones and conglomerates. These rock members represent ancient, freshwater lakes, rivers, streams and sand dunes.

My third skull, most of the rostrum is restored. However, I restored it a little too thickly. All teeth are replicas. This jaw doesn’t go with this skull, I found the skull about 100m downstream and decided to match them. I think it is an accurate fit. The back jaw hinges are restored along with most of the teeth, except for the darker teeth, which are authentic.

Most commonly, I find Phytosaur and Ateosaur fossils consisting of teeth, limb bones, vertebrae and scutes. I only collect fossils that have been exposed to the elements but, unfortunately, I rarely find any articulated bones. This is undoubtedly due to the fact that these fossils are usually found in formations created by ancient streams and rivers. These streams and rivers evidently scattered bone more often than they buried intact skeletons. Occasionally, rapid burial by these streams did occur, resulting in well-preserved, individual bones and skulls being found. I have found incomplete but fairly well preserved Phytosaur skulls and these are among my favourite fossils.

 I have excavated, stabilised and restored four Phytosaur skulls. The skulls are Rutiodontinae, from the genus Leptosuchus, but one of which is possibly from the genus Smilosuchus. These are robust forms of Phytosaur. Some of these phytosaurs grew quite large, perhaps 7m to 8m in length and, though not closely related, can be thought of as the Triassic version of our modern-day crocodiles.

In addition to the previously mentioned fossils, I occasionally find rauisuchian (large-skulled archosaurs) teeth, dinosaur limbs and teeth, lungfish teeth, Metoposaur (large, predatory Amphibians) bones, Fern imprints, Horsetail imprints and petrified logs. I have also found small coprolites in one of the Triassic lake deposits. One of my honor classes slowly dissolved a number of these coprolites in test tubes using acetic acid. In most cases, the coprolites completely dissolved away and left nothing. But in a small percentage of cases, we found tiny, semionotids-like fish scales and bones. One specimen had what appeared to be a pincher claw, perhaps from a crustacean.

I have donated the majority of my finds to small, local museums and interpretive centers. I also keep some fossils in my classroom where they are displayed and used in teaching.

We have a great store of Triassic-age history here in the rocks of West Texas. Much of this history is poorly known and much of our Triassic areas are, palaeontologically speaking, unexplored. This is due to the fact that the majority of these Canyonlands exist within privately owned ranches with landowners who typically do not allow visitors and the terrain is rugged and not easily travelled.  For those who do get the opportunity to investigate this terrain, it is difficult not to imagine a great fossil discovery waiting to be made just over the next hill.